In a typical thermal printer, a web-type carrier containing a repeating series of spaced frames of different colored heat transferable dyes is spooled on a carrier supply spool. The carrier is paid out from the supply spool and rewound on a take-up spool. It moves through a nip formed between a thermal print head and a dye-absorbing receiver. The receiver is in turn supported by a platen in the form of a drum. The print head engages the carrier and presses it against the receiver. The receiver may for example be coated paper and the print head is formed of, for example, a plurality of heating elements. When a particular heating element is energized, it is heated. In the presence of heat and pressure, dye from the carrier is caused to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the carrier. These types of thermal printers offer the advantage of "true continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel on the receiver.
The web-type carrier often includes a repeating series of spaced yellow, magenta and cyan dye frames. The carrier is typically formed of a very thin, flexible dye carrying member having a thickness that can be on the order of 1/4 mil. First, the yellow frame in the carrier is moved to a position under the print head. The print head is lowered to apply pressure on the carrier and while the print platen is advanced, the heating elements are selectively energized to form a row of yellow image pixels in the receiver, which is moved by the drum under the print head. This printing of the yellow dye is repeated line-by-line until the entire yellow dye image is formed in the receiver. Next, the magenta carrier frame is moved under the print head. The receiver is advanced to align the starting point of the yellow dye image with the print head heating elements. The print head is lowered and both the receiver and the magenta carrier frame are simultaneously moved through the nip as the heating elements are selectively energized and a magenta image is formed superimposed upon the yellow image. Finally the cyan dye carrier frame and the receiver dye image starting point are moved under the print head, and the heating elements are again selectively energized so that a cyan dye image is formed in the receiver superimposed upon the yellow and magenta dye images. The yellow, magenta and cyan dye images combine to form a color image. The print head must apply uniform pressure across the width of the receiver in order to provide a high quality print. Several factors can cause a nonuniform pressure to be applied as the head loads the carrier against the receiver. For example, because of the differences in tolerances in manufacture of the head and the receiver, there is often not a uniform pressure applied across the width of a receiver by a head as it loads or presses a carrier against the receiver. This can adversely affects printing quality.
Another problem is concerned with wear of the lifting mechanism. As the carrier is advanced through the nip position, the print head is raised. It is lowered during printing. This should be accomplished with precision and repeatability to insure color image registration. The print head lifting mechanism should not wear or it may cause non-uniform print density.